3D printing of high stiffness-to-weight reflective optics

ABSTRACT

A 3D-printed reflective optic providing very high specific stiffness through the utilization of a hollow shelled design, with closed back, filled with high-stiffness internal volumetric space-filling open-cell lattice structures. High-stiffness, structurally-integrated, sacrificial structures are also included for the purposes of reduction or elimination of tooling during post-processing operations.

FIELD

The disclosure relates to high stiffness-to-weight reflective optics,and more particularly, to 3D printing of a lattice structure within amirror cavity to reduce or eliminate tooling during post-processingoperations.

BACKGROUND

Many optical applications require mirrors designed to maintainexceptional image quality under high inertial loading scenarios thatoccur during operation. Known optical structures are comprised ofberyllium alloy mirrors with machined waffle lightweighting featuresbecause of the materials' unique properties for high specific stiffness(i.e. stiffness-to-weight ratio). However, their fabrication oftenrequires expensive tooling and facilities requirements usuallyassociated with diamond machining of optics and handling of toxicmaterials. As a result, cost, availability, and typical lead times forhigh-precision beryllium alloy components are also disadvantages ofusing these high-performance materials.

What is needed is a device, system, and method to provide reflectiveoptics with comparable stiffness-to-weight performance and at a lowercost compared to known beryllium alloy mirrors.

SUMMARY

An embodiment provides a 3D printed high stiffness-to-weight reflectiveoptic comprising an internal mirror cavity located within an exteriormirror shell comprising a closed-face back; the internal mirror cavitycomprising a 3-dimensional space-filling volumetric lattice structure;and a plurality of 3D-printed sacrificial integrated structures printedon the close-face back, wherein the 3D-printed sacrificial integratedstructures have two or more legs, wherein each the leg comprises anindividually frangible segment whereby a top planar surface of each thesacrificial integrated structure is adequately supported by all legs forresisting machining stresses, and each leg is individually severable,whereby each the sacrificial integrated structure is removable uponindividual severing of each leg frangible section. In embodiments the 3Dprinted sacrificial integrated strictures are tripods, the tripodscomprising legs with varied lengths and the top planar surface. Otherembodiments comprise a powder metal material callout of an AlSi10Mgalloy. In subsequent embodiments the lattice comprises a printed latticetopology selected from triangular hybrid, truncated octahedron, gyroids,cubic truss, octet truss, truncated tetrahedron, and Archimedean solids.For additional embodiments the lattice comprises an open-cell tetragonalprinted lattice topology. In another embodiment, the lattice comprisestetragonal unit cells of about 0.40 inch by about 0.40 inch. For afollowing embodiment, the lattice comprises struts of about 0.040 inchdiameter. In subsequent embodiments the mirror has a surface qualitygreater than or equal to about 64 Root Mean Square (RMS). In additionalembodiments the stiffness to weight ratio comprises a specific stiffnessof about 150 E/ρ and a Young's modulus of elasticity of about 300 GPa.

Another embodiment provides a method for fabricating a 3D printed highstiffness-to-weight reflective optic comprising defining reflectiveoptical specifications for a surface of the reflective optic;investigating a lattice for a 3-dimensional space-filling volumetriclattice structure to support the reflective optic; creating an assemblymodel from results of the lattice investigation; printing a mirrorassembly from the assembly model, the assembly comprising a plurality of3D-printed sacrificial integrated support structures printed on aclose-face back of the mirror assembly, wherein the 3D-printedsacrificial integrated structures have two or more legs; performing aheat treatment on the printed mirror assembly; machining a mirrorsurface on the surface of the reflective optic; and removing ForeignObject Debris comprising removing the plurality of sacrificialintegrated support structures. In included embodiments the latticeinvestigation comprises the step of determining weight and stiffness bylattice analysis. In yet further embodiments further comprising thesteps of creating a latticed mirror model and assessing printability ofthe assembly model between the steps of investigating a lattice andcreating an assembly. Related embodiments further comprise the steps ofvalidating Finite Element Analysis (FEA) of the assembly model by taptesting and creating an Additive Manufacturing (AM) mirror drawingbetween the steps of investigating a lattice and creating an assembly.For further embodiments creating the assembly comprises the step oflocating a mirror cavity in an exterior mirror shell with 0.030 inchinterference fit between the exterior mirror shell and the mirrorcavity. In ensuing embodiments the step of printing the mirror assemblycomprises a powder metal material callout of an AlSi10Mg alloy. For yetfurther embodiments, the step of removing the plurality of sacrificialintegrated support structures comprises severing an individuallyfrangible segment of each leg of each sacrificial integrated supportstructure whereby each the sacrificial integrated support structure isremoved. For more embodiments, the step of performing heat treatmentcomprises the steps of Hot Isostatic Press (HIP); and Solution HeatTreatment. In continued embodiments the step of machining a mirrorsurface comprises the steps of rough machining, stress relief, finalmachining, solution heat treatment, and finishing. For additionalembodiments, the step of machining the mirror comprises a surfacequality greater than or equal to about 64 Root Mean Square (RMS).

A yet further embodiment provides a 3D printed high stiffness-to-weightreflective optic comprising providing the mirror by defining reflectiveoptical specifications for a surface of the reflective optic;investigating a lattice for a 3-dimensional space-filling volumetriclattice structure to support the reflective optic; creating an assemblymodel from results of the lattice investigation; printing a mirrorassembly from the assembly model, the assembly comprising a plurality of3D-printed sacrificial integrated support structures printed on aclose-face back of the mirror assembly, wherein the 3D-printedsacrificial integrated structures have two or more legs; performing aheat treatment on the printed mirror assembly; machining a mirrorsurface on the surface of the reflective optic; and removing ForeignObject Debris comprising removing the plurality of sacrificialintegrated support structures, the step of removing the plurality ofsacrificial integrated support structures comprising severing anindividually frangible segment of each leg of each sacrificialintegrated support structure; wherein dimensions of the surface of thereflective optic are about 9.75 by 5.8 inches, and a surface quality isgreater than or equal to about 64 Root Mean Square (RMS).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a reflective optics application configured inaccordance with an embodiment.

FIG. 2 is a generalized method for manufacturing reflective opticsconfigured in accordance with an embodiment.

FIG. 3 depicts a reflective optics lattice structure configured inaccordance with an embodiment.

FIG. 4 is a reflective optics back perspective view configured inaccordance with an embodiment.

FIG. 5 is a reflective optics front perspective view configured inaccordance with an embodiment.

FIG. 6 is a reflective optics back plan scale view configured inaccordance with an embodiment.

FIG. 7 is a reflective optics A-A cross section scale view configured inaccordance with an embodiment.

FIG. 8 is a reflective optics side scale view configured in accordancewith an embodiment.

FIG. 9 is a scanning mirror front view configured in accordance with anembodiment.

FIG. 10 is a scanning mirror back view configured in accordance with anembodiment.

FIG. 11 is a scanning mirror side view configured in accordance with anembodiment.

FIG. 12 is a scanning mirror end view configured in accordance with anembodiment.

FIG. 13 is a flow chart for a reflective optics fabrication methodconfigured in accordance with an embodiment.

These and other features of the present embodiments will be understoodbetter by reading the following detailed description, taken togetherwith the figures herein described. The accompanying drawings are notintended to be drawn to scale. For purposes of clarity, not everycomponent may be labeled in every drawing.

DETAILED DESCRIPTION

The features and advantages described herein are not all-inclusive and,in particular, many additional features and advantages will be apparentto one of ordinary skill in the art in view of the drawings,specification, and claims. Moreover, it should be noted that thelanguage used in the specification has been selected principally forreadability and instructional purposes, and not to limit in any way thescope of the inventive subject matter. The invention is susceptible ofmany embodiments. What follows is illustrative, but not exhaustive, ofthe scope of the invention.

FIG. 1 illustrates a reflective optical application 100. In thisembodiment, scanning mirror reflective optic 105 is a component of LightDetection and Ranging (LIDAR) system 110. LIDAR system 110 is employed,for example, in airborne detection of mines 115. The scanning mirror forsuch missions is a high precision highly reflective surface with highstiffness-to-weight ratio to maintain image quality. The fast scanningmotion induces high inertial loading and causes distortion if the mirroris not adequately stiff. In operation of the LIDAR system, the mirroroperates on a 2 or more axis scan assembly and reflects laser energy inorder to illuminate an object or surface such that the reflection isthen measured and processed. Most recently, LIDAR systems are also beingused in autonomous vehicles. In some high performance applications, thescanning mirror is made of beryllium or a beryllium alloy due to itscharacteristics for creating a stiff surface.

Replacement of an exotic (beryllium alloy), high performance mirrormaterial with a conventional lower performance material (aluminum alloy)while preserving its overall structural performance and systemrequirements is exceedingly difficult with conventional techniques.According to one example, the efficient structural design is fabricatedby additive manufacturing (AM), also referred to herein as 3D printingwhich includes a number of processes including direct metal lasersintering (DMLS) among others. In embodiments, sacrificial tripods are“grown” directly into the back surface of the mirror to be used astemporary fixturing for diamond machining process. These are 3D printedon the structure, and removed after the diamond turning process. Thistechnique eliminates the need for expensive tooling usually associatedwith diamond machining of optics. In addition, specific heat treatmentmethods attain fine grain structure and dimensional stability requiredfor production of optical quality mirrors.

Additive manufacturing allows the use of internal lattice structures andhollow core construction not possible with conventional machining.Embodiments use 3D printed features (tripods) on the back of the part toaid in the final machining. The tripods provide built in support for thepart for diamond turning of the mirror to final required specifications.

As noted, beryllium alloy mirrors are extremely expensive structures tomachine. In contrast, 3D printing of a lattice structure within a mirrorcavity that is designed to have the same approximate stiffness to weightratio as the traditional beryllium alloy design (a specific stiffness ofabout 150 E/ρ and a Young's modulus of elasticity of about 300 GPa inembodiments) greatly reduces the cost of manufacturing withoutcompromising performance. In embodiments, the lattice is an open-celllattice. In addition, tripods grown into the back of the mirror allowfor diamond turning. Without these tripods, significant added cost wouldbe incurred. This technique can be employed for many other applicationsthat use additive manufactured parts, but is especially effective onoptical mirror fabrication.

In one example, the final lattice geometry can be optimized byconducting a series of printability and structural modal testingassessments. Sample portions of the lattice in the present system wereadditively manufactured and qualitatively evaluated for characteristicssuch as warping, unconnected lattice members, geometric anomalies, modalresponse, and dimensional accuracy.

In embodiments, the diameter of the lattice strut members affected theoverall printability of the model as the natural frequency is verysensitive to lattice strut diameter.

Embodiments employ a tetragonal truss printed lattice topology for themirror. Many options are also possible such as triangular hybrid,truncated octahedron, gyroids, cubic truss, octet truss, truncatedtetrahedron, and other Archimedean solids. Design criteria includeweight, stiffness, and strength. In embodiments, unit cell sizeselection is important to decrease the size of the unsupportedhorizontal regions.

FIG. 2 is a generalized method for manufacturing reflective optics 200according to one embodiment of the present technique. Steps comprisedefining reflective optical specifications 205 for the specificapplication and design criteria. For example, the size, weight,stiffness, and reflectivity are some of the variables that are used forthe specification. The lattice investigation including topology 210 isconsidered in order to design the optimal lattice geometry. There arevarious simulation tools Computer Aided Design and Finite ElementAnalysis (CAD and FEA) such as Creo, Netfabb and NX that facilitate thedesign phase and allow iterations to better define the lattice. Suchtools enable a lattice analysis to assess the weight and stiffnessparameters. In one example, lattice finite element analysis (FEA) wascreated for the lattice topology and ran as a free-free modal andcompared the natural frequencies. Parameters include size (cubic), celldimensions, strut thickness, Young's modulus, and density. Inembodiments the software tool importantly incudes each of: Organictopology optimization, STL File Repair, Part Orientation and SupportOptimization, Multi-Physics AM Process Simulation, and Build FailurePrediction. Netfabb includes these. These are important because creatinglattice geometries with high stiffness-to-weight ratio must be anefficient and reliable process to enable rapid design iteration. Forexample, creating lattice strut members as individual Creo part filesand patterning the geometry has several drawbacks. It is nearlyimpossible to update or modify the lattice topology, it is verydifficult to change connections with solid body features withoutstarting over with the design, and it is very inefficient for iterativedesign and optimization. Further, it is impossible to model structuressuch as the triangular hybrid, the Bucky ball, the tetragonal, and theoctet truss with 3D elements due to element count and run time. Forembodiments, strut members are analyzed as 1D FEA beam elements,reducing the element count, making analysis possible.

Once the lattice structure meets the requirements, the mirror model withthe internal lattice features is created 215. According to one example,two parts were created to lattice topology. One part is the exteriormirror shell and the other is the mirror cavity. Once combined, theshell and the cavity are completed and the deign build is exported, forexample as an .stp for Netfabb.

Once the build file is imported into the build environment, the solidmirror cavity section can be built. The desired unit lattice cell wasgenerated and then patterned inside the mirror cavity. Once the cavitywas latticed, the lattice can be positioned within the mirror shell andaligned.

The steps continue with printing mirror assembly (mirror cavity andmirror shell with tripods) 220; performing heat treatment 225; machiningmirror 230; and tripod and Foreign Object Debris (FOD) removal 235.According to one embodiment, the reflective optic is subject to acoating 240. The coating helps preserve the reflective properties.

FIG. 3 depicts a reflective optics lattice structure 300. Inembodiments, tetragonal lattice dimensions comprise strut diameter 305of 0.040 inch; unit-cell dimensions of 0.40 inch 310 by 0.40 inch 315.In other embodiments, unit cells based on truncated octahedrongeometries comprise 0.040″ strut dimensions and 0.50″ unit celldimensions. These dimensions provide the required stiffness propertiesfor the mirror function. As mentioned, for embodiments, the lattice isan open-cell lattice.

FIG. 4 is a reflective optics back perspective scale view 400.Illustrated are tripods 405 (one end only) and build direction 410.Depicted tripods are sacrificial structures comprising legs 415 and topsurface 420. In embodiments, top surfaces 420 are planar to providerequired support for machining, such as diamond turning of the opticalsurface. By having two or more legs having a thinner dimension ascompared to the total size of the top surface, the structures are easilyremoved such as by simple mechanical tools. In embodiments, legs 415comprise frangible sections 425 to enable removal. Due to leg frangiblesections 425, sacrificial structures (tripods) can be removed by simplemeans such as manually with wire cutters. No specialized tooling ormachining is required to remove them. The number and location ofstructures 405 are established for the particular mirror assembly tominimize deflections in the part during optical finishing, which iscritical to getting good surface quality in the first few passes versusmultiple passes. This results in better surface figure and finishcompared to mirrors having only one-tenth the surface area. In oneexample an equal number of structures are provided on both side sectionsof the back side, with no structures in the middle section. In oneexample there are between 4-8 structures on either side portion.

FIG. 5 is a reflective optics front perspective scale view 500.

FIG. 6 is a reflective optics back plan scale view 600. Embodimentdimensions are overall length 9.75 inches (2) and width 5.81 inches (3).

FIG. 7 is a reflective optics 1-1 cross section scale view 700 of FIG. 6. Internal volumes are filled with lattice structure (not shown). Asnoted, sacrificial structures 405 have legs 415, with frangible sections425, of varying lengths so that the top surface 420 of structures 405are approximately planar.

FIG. 8 is a reflective optics side scale view 800. Embodiment dimensionscomprise a front to back thickness of 1.32 inches (4) and 1.58 inches(5) face to tripod surface.

FIG. 9 is a scanning mirror embodiment front view 900.

FIG. 10 is a scanning mirror embodiment back view 1000.

FIG. 11 is a scanning mirror embodiment side view 1100.

FIG. 12 is a scanning mirror embodiment end view 1200.

FIG. 13 is a flow chart for a reflective optics fabrication methodembodiment 1300. Steps comprise defining reflective opticalspecifications 1305, which can include, for example, size, weight,stiffness, and reflectivity, depending on specific application anddesign criteria; lattice analysis 1310, including topology, determiningweight and stiffness for the optimal lattice geometry. Following thelattice investigation, creating latticed mirror model 1315, with theinternal lattice features. Embodiment mirror models comprise two parts,the exterior mirror shell, and the mirror cavity. The created mirrormodel is assessed for printability, including build direction, supportstructure location, and machine process settings 1320. In embodimentsnext is validating FEA by tap testing 1325. Then creating additivemanufacturing mirror drawings 1330. In embodiments this is followed bycreating the assembly by locating the mirror cavity in the exteriormirror shell with, in embodiments, a 0.030 inch interference fit betweenthe shell and the cavity 1335; printing the mirror assembly (mirrorcavity and mirror shell with tripods) having a powder metal materialcallout of AlSi10Mg alloy 1340. The 3D printed sacrificial integrated(tripod) structures are printed on the close-face back. Next areperforming heat treatment comprising stress relief, Hot Isostatic Press(HIP), and Solution Heat Treatment 1345; machining the mirror comprisingrough machining, stress relief, final machining, solution heattreatment, and finishing 1350; Foreign Object Debris (FOD) removal 1355;and optional coating 1360 to protect the reflective properties of themirror surface. The 0.030 inch interference fit is important as beingsufficient for making solid lattice connections to the shell. Inembodiments, step 1335, FOD removal, comprises severing each leg of eachsacrificial support structure, thereby removing them without specializedtooling or machining. Each leg comprises an individually frangiblesegment whereby the planar surface is adequately supported by all legsfor resisting machining stresses, and each leg is individuallyseverable. This reduces the required tooling and fixturing. Asmentioned, the sacrificial support structures are established for theparticular mirror assembly to minimize deflections in the part duringoptical finishing, which is critical to getting good surface quality inthe first few passes versus multiple passes. This results in bettersurface figure and finish compared to mirrors having only one-tenth thesurface area. One measure, RMS, is the Root Mean Square average of theprofile height deviations from the mean line, recorded within theevaluation length (see ASME B46.1). In embodiments, RMS is greater thanor equal to about 64 RMS.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the blocks may occur out of theorder noted in the Figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

The foregoing description of the embodiments has been presented for thepurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of this disclosure.It is intended that the scope of the present disclosure be limited notby this detailed description, but rather by the claims appended hereto.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the scope of the disclosure. Although operations are depicted inthe drawings in a particular order, this should not be understood asrequiring that such operations be performed in the particular ordershown or in sequential order, or that all illustrated operations beperformed, to achieve desirable results.

Each and every page of this submission, and all contents thereon,however characterized, identified, or numbered, is considered asubstantive part of this application for all purposes, irrespective ofform or placement within the application. This specification is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Many modifications and variations are possible in light ofthis disclosure. Other and various embodiments will be readily apparentto those skilled in the art, from this description, figures, and theclaims that follow. It is intended that the scope of the invention belimited not by this detailed description, but rather by the claimsappended hereto.

What is claimed is:
 1. A 3D printed high stiffness-to-weight reflectiveoptic comprising: an internal mirror cavity located within an exteriormirror shell comprising a closed-face back; said internal mirror cavitycomprising a 3-dimensional space-filling volumetric lattice structure;and a plurality of 3D-printed sacrificial integrated structures printedon said close-face back, wherein said 3D-printed sacrificial integratedstructures have two or more legs, wherein each said leg comprises anindividually frangible segment whereby a top planar surface of each saidsacrificial integrated structure is adequately supported by all legs forresisting machining stresses, and each leg is individually severable,whereby each said sacrificial integrated structure is removable uponindividual severing of each leg frangible section.
 2. The device ofclaim 1 wherein said 3D printed sacrificial integrated strictures aretripods, said tripods comprising legs with varied lengths and said topplanar surface.
 3. The device of claim 1 comprising a powder metalmaterial callout of an AlSi10Mg alloy.
 4. The device of claim 1 whereinsaid lattice comprises a printed lattice topology selected fromtriangular hybrid, truncated octahedron, gyroids, cubic truss, octettruss, truncated tetrahedron, and Archimedean solids.
 5. The device ofclaim 1 wherein said lattice comprises an open-cell tetragonal printedlattice topology.
 6. The device of claim 1 wherein said latticecomprises tetragonal unit cells of about 0.40 inch by about 0.40 inch.7. The device of claim 1 wherein said lattice comprises struts of about0.040 inch diameter.
 8. The device of claim 7 wherein said mirror has asurface quality greater than or equal to about 64 Root Mean Square(RMS).
 9. The device of claim 1 wherein said stiffness to weight ratiocomprises a specific stiffness of about 150 E/ρ and a Young's modulus ofelasticity of about 300 GPa.
 10. A 3D printed high stiffness-to-weightreflective optic comprising: providing a mirror by: defining reflectiveoptical specifications for a surface of said reflective optic;investigating a lattice for a 3-dimensional space-filling volumetriclattice structure to support said reflective optic; creating an assemblymodel from results of said lattice investigation; printing a mirrorassembly from said assembly model, said assembly comprising a pluralityof 3D-printed sacrificial integrated support structures printed on aclose-face back of said mirror assembly, wherein said 3D-printedsacrificial integrated structures have two or more legs, wherein eachsaid leg comprises an individually frangible segment whereby a topplanar surface of each said sacrificial integrated structure isadequately supported by all legs for resisting machining stresses;performing a heat treatment on said printed mirror assembly; machining amirror surface on said surface of said reflective optic; and removingForeign Object Debris comprising removing said plurality of sacrificialintegrated support structures, said step of removing said plurality ofsacrificial integrated support structures comprising severing anindividually frangible segment of each leg of each sacrificialintegrated support structure; wherein dimensions of said surface of saidreflective optic are about 9.75 by 5.8 inches, and a surface quality isgreater than or equal to about 64 Root Mean Square (RMS).